In the trend toward downsizing and entire solidification in the field of ionics, demands are increasing for the practical use of an entire solid primary battery, secondary battery or electric double layer capacitor using a solid electrolyte as a new ion conductor which can substitute for the conventional electrolyte solution.
More specifically, batteries with a conventional electrolyte solution readily undergo the occurrence of liquid leakage or elution of the electrode material outside the battery and have a problem in the long-term reliability. Flexible sheet batteries which are hoped for in recent years also have a problem in that when an electrolyte solution is used, the internal impedance elevates or internal short circuit occurs due to the localization of electrolyte solution within the battery container or to the exhaustion of liquid.
Recently, electric double layer capacitors using a carbon material having a large specific surface area as the polarizable electrodes and placing an ion conductive solution therebetween are used in many cases as a power source for memory backup. However, such an electrolytic double layer capacitor also has a problem in the long-term use or reliability because when it is used for a long period of time or when a high voltage is applied, liquid leakage outside the capacitor readily occurs. On the other hand, electric double layer capacitors using a conventional inorganic ion conductive material further have a problem in that the decomposition voltage of the ion conductive material is low and, hence, the output voltage is low.
Batteries and electric double layer capacitors using a solid polymer electrolyte are free of problems such as liquid leakage or elution of the electrode material and can be processed into various shapes or easily sealed. They are also easy to be more reduced in the thickness.
Further, it is reported that in the electric double layer capacitor using a polyphosphazene-based organic polymer as the main component of the ion conductive material, the output voltage elevates as compared with those using an inorganic ion conductive material (see, for example, JP-A-4-253771 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")).
Although solid polymer electrolytes under study in general are improved in the ion conductivity up to approximately from 10.sup.-4 to 10.sup.-5 S/cm at room temperature, this still stays in a level by two figures lower than that of solution-based ion conductive materials. The same applies to solid polymer electrolytes having introduced thereinto an oligooxyethylene chain, which are given much attention in recent years (see, for example, JP-A-4-211412 corresponding to U.S. Pat. No. 5,194,490). Further, there is a problem in that at low temperatures of 0.degree. C. or less, the ion conductivity generally lowers to an extreme extent.
For installing a solid electrolyte into a battery or electric double layer capacitor, a so-called cast method have been used, where a solid electrolyte solution is coated and spread on a substrate such as an electrode and then the solvent is evaporated and removed. However, this technique is disadvantageous in that the working operation is complicated and adhesion to the electrode is unsatisfactory. There have been proposed a method of using a polymer gel electrolyte and a method of using a cross-linked solid polymer electrolyte impregnated with an electrolyte solution (see, for example, U.S. Pat. No. 4,792,504). However, when a large amount of electrolyte solution is contained in order to obtain a satisfactory ion conductivity, the curability or film forming property is deteriorated and the film strength is insufficiently high. Further, due to the fluidity imparted to it, the electrolyte cannot be treated as a complete solid and when it is applied to an electric double layer capacitor or battery, short circuit readily occurs and there arises a problem in the sealing property similarly to the solution-based ion conductive material.
Accordingly, there has been made investigation on a curing method in which an electrolyte and a polymerizable compound are used as the main components of the solid polymer electrolyte and these are loaded in a structural body of a battery or capacitor in the liquid or gel form and then cured to effect compounding.
As such a curing method of a polymerizable composition, there has hitherto been aggressively investigated for development of a curing method using an actinic radiation, and in particular, study is being made of solid polymer electrolytes to be prepared using an ultraviolet polymerization initiator, which is economically advantageous. However, when use is made of exposure to a radiation, it is difficult, due to the construction of the battery, to simultaneously compound and integrate the respective elements of the battery, i.e., a positive electrode, a negative electrode and/or a separator, as well as a polymerizable composition for the solid electrolyte. Particularly, in batteries of the type in which a positive electrode, a solid electrolyte and a negative electrode are laminated or wound, the elements are each not light transmissive and are difficult to be integrated. In order to prevent curing failure due to the transmission incapability of actinic radiation, it may be considered to compound the elements, i.e., the positive electrode and the negative electrode, and the polymerizable composition for solid electrolyte separately and then laminate or otherwise integrate them. However, problems occur in that the actinic radiation is shielded by the electrode material and the polymerizable composition inside the electrode is insufficiently cured so that the polymerization proceeds unevenly in the depth direction of the electrode or in that when a separator is interposed which is used in compensating for the mechanical strength or inter-electrode gap, uniform curing across and to the backside of the separator is difficult to attain. A further problem is involved in that the polymerization is vulnerable to inhibition by oxygen contained in the atmosphere which the polymerizable composition contacts, thereby causing curing failure.
For this reason, a curing method by heat curing has also been proposed, in which the respective elements, i.e., the positive electrode, the negative electrode and/or the separator and the solid electrolyte can be compounded and integrated simultaneously with the curing, and the construction of the battery allows reduction in the internal impedance of the battery. This method gains an advantage over any other methods for batteries of the type in which a positive electrode, a solid electrolyte and a negative electrode are laminated or wound, that are difficult to make by a photocuring method with an actinic radiation. However, the problems as described below would occur. That is, in the preparation of a polymerizable composition for solid polymer electrolytes using a thermopolymerization initiator, the initiator is in many cases selected depending on the desired curing temperature. Accordingly, when the electrolyte solution contains a low boiling point solvent, use of initiators which generate radicals at high temperatures is restricted so as to prevent changes in the composition of solution due to the evaporation of the solvent. Consequently, it is attempted to use a polymerization accelerator in combination so that curing can be performed at a temperature of from room temperature to a medium temperature. The polymerization accelerator (in many cases, a reducing agent) or decomposition products thereof, however, will deteriorate the current properties such as ion conductivity or the characteristics such as cycle life, of the solid polymer electrolyte. If curing is performed only by heating without using any polymerization accelerator, it takes a long time for the curing to be completed at low temperatures since the curing rate depends on the thermal decomposition rate of the thermopolymerization initiator.
It is a common technique to increase the amount of the polymerization initiator or radicals generated so as to efficiently perform curing. However, unreacted initiator or decomposition products thereof increase in quantity and adversely affect the current properties such as ion conductivity or electrochemical characteristics such as cycle life. Furthermore, when the upside and/or inside of the electrode is compounded with a solid electrolyte, depending on the kind of the polymerization initiator, a problem occurs in that a gaseous decomposition product is generated and due to the gas, the electrode material may come off from the collector or the electrode may expand, thereby causing changes in the battery shape, or the electrochemical properties may be adversely affected, for example, the surface resistance increases or the cycle properties are deteriorated.
Use of peroxydicarbonate as a thermopolymerization initiator has also been proposed to solve the problem of gas generation (see, for example, JP-A-6-203841). However, a thermopolymerizable composition capable of exhibiting excellent electrical conductivity even at a low temperature and providing a cured product having sufficiently high strength and flexibility is not yet known.